1. Field of the Invention
This invention relates to low melting, opaque enamel frits free of lead by which they are harmless against human body and involve no ecological problems in or during the manufacturing or application thereof
2. Description of the Prior Art
One of the drawbacks involved in enameled metal wares is deformation thereof owing to the strain of steel plate bodies produced on firing. It is known that this strain is suppressed to a lesser extent at lower firing temperatures and no occurrence of the strain is experienced at temperatures below 723.degree. C. which is the A.sub.1 transformation point of steel plate. It is also known that low temperature firing will reduce defects of enameled wares produced by gases such as, for example, bubbles, pinholes, fishscales and the like. Moreover, recent rising cost of petroleum results in a relatively large proportion occupied by fuel cost in production cost of enameled wares. Therefore, there is a strong demand of enamels, which can be fired at low temperatures, from the standpoint of resource and energy savings.
As such is the case, various attempts have been made in Japan and abroad to manufacture low melting enamels comprising PbO or P.sub.2 O.sub.5 as their main component. However, these enamels have ecological, safety and cost problems coupled with the fact that they are not so excellent in their characteristics as to take the place of conventional enamels. Thus, the known enamels are not satisfactory.
Currently used opaque enamels whose firing temperatures are over 800.degree. C. can be broadly divided into two classes. One of the classes includes addition of titania at the time of mill addition. The other class includes use of TiO.sub.2, zirconia and Sb.sub.2 O.sub.3 as glass components and formation of opaque crystals on firing, which and may sometimes be called a titania opacified enamel.
Although these classes have the respective merits and demerits, the latter class is superior in luster, hiding power and whiteness.
Typical of known titania glazes for the titania opacified enamel frit are those which have a composition comprising 47.5 wt. % of SiO.sub.2 (hereinafter referred to simply as %), 12.8% of B.sub.2 O.sub.3, 11.9% of Na.sub.2 O, 2.9% of K.sub.2 O, 4.2% of F.sub.2, 2.7% of P.sub.2 O.sub.5, and 18.0% of TiO.sub.2. The whiteness and opacity of the titania glaze is greatly influenced by the fundamental composition of glass even though the firing temperature is held constant. In other words, compositions having the same components greatly differ from one another in whiteness and opacity when the ratios of the components vary. It is known from experience that (1) adequate opacity cannot be obtained unless a B.sub.2 O.sub.3 /Na.sub.2 O ratio is high; (2) if SiO.sub.2 is decreased or if Na.sub.2 O is increased, the resulting glaze assumes brown color; (3) addition of Al.sub.2 O.sub.3 in small amounts results in the increase of opacity; and (4) where CaO, ZnO and MgO are added at levels of 5%, MgO has a slight effect on enhancement of opacity but the other components produce no effect.
As will be apparent from the experience of the known opaque enamels, even though compositions comprising no specific materials are used, the resulting frits will produce different characteristics depending on the ratio or manner of combination of the materials. Even a slight difference in composition may not bring out an intended, beautiful, white color. Accordingly, so far as the characteristics of frit are concerned, apparently similar frit compositions may be completely different from each other.